There is known a sewing method of material in which material edges are stitched together with using a flat seamer sewing machine having four or five needles and with six threads (e.g. refer to Patent Document 1). According to the sewing method using the flat seamer sewing machine, since the material edges are stitched together in the abutted state, the unevenness in a stitched portion can be reduced. In addition, since the material edges are stitched together with six threads, the strength of the stitched portion can be ensured.